/*
 * Copyright (C) 2013 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import static com.google.common.base.Preconditions.format;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import javax.annotation.Nullable;

/**
 * Static convenience methods that serve the same purpose as Java language
 * <a href="http://docs.oracle.com/javase/7/docs/technotes/guides/language/assert.html">
 * assertions</a>, except that they are always enabled. These methods should be used instead of Java
 * assertions whenever there is a chance the check may fail "in real life". Example:
 * 
 * <pre>
 * {
 *     &#64;code
 *
 *     Bill bill = remoteService.getLastUnpaidBill();
 *
 *     // In case bug 12345 happens again we'd rather just die
 *     Verify.verify(bill.status() == Status.UNPAID, "Unexpected bill status: %s", bill.status());
 * }
 * </pre>
 *
 * <h3>Comparison to alternatives</h3>
 *
 * <p>
 * <b>Note:</b> In some cases the differences explained below can be subtle. When it's unclear which
 * approach to use, <b>don't worry</b> too much about it; just pick something that seems reasonable
 * and it will be fine.
 *
 * <ul>
 * <li>If checking whether the <i>caller</i> has violated your method or constructor's contract
 * (such as by passing an invalid argument), use the utilities of the {@link Preconditions} class
 * instead.
 *
 * <li>If checking an <i>impossible</i> condition (which <i>cannot</i> happen unless your own class
 * or its <i>trusted</i> dependencies is badly broken), this is what ordinary Java assertions are
 * for. Note that assertions are not enabled by default; they are essentially considered "compiled
 * comments."
 *
 * <li>An explicit {@code if/throw} (as illustrated below) is always acceptable; we still recommend
 * using our {@link VerifyException} exception type. Throwing a plain {@link RuntimeException} is
 * frowned upon.
 *
 * <li>Use of {@link java.util.Objects#requireNonNull(Object)} is generally discouraged, since
 * {@link #verifyNotNull(Object)} and {@link Preconditions#checkNotNull(Object)} perform the same
 * function with more clarity.
 * </ul>
 *
 * <h3>Warning about performance</h3>
 *
 * <p>
 * Remember that parameter values for message construction must all be computed eagerly, and
 * autoboxing and varargs array creation may happen as well, even when the verification succeeds and
 * the message ends up unneeded. Performance-sensitive verification checks should continue to use
 * usual form:
 * 
 * <pre>
 * {
 *     &#64;code
 *
 *     Bill bill = remoteService.getLastUnpaidBill();
 *     if (bill.status() != Status.UNPAID) {
 *         throw new VerifyException("Unexpected bill status: " + bill.status());
 *     }
 * }
 * </pre>
 *
 * <h3>Only {@code %s} is supported</h3>
 *
 * <p>
 * As with {@link Preconditions} error message template strings, only the {@code "%s"} specifier is
 * supported, not the full range of {@link java.util.Formatter} specifiers. However, note that if
 * the number of arguments does not match the number of occurrences of {@code "%s"} in the format
 * string, {@code Verify} will still behave as expected, and will still include all argument values
 * in the error message; the message will simply not be formatted exactly as intended.
 *
 * <h3>More information</h3>
 *
 * See <a href="https://github.com/google/guava/wiki/ConditionalFailuresExplained">Conditional
 * failures explained</a> in the Guava User Guide for advice on when this class should be used.
 *
 * @since 17.0
 */
@Beta
@GwtCompatible
public final class Verify {
    /**
     * Ensures that {@code expression} is {@code true}, throwing a {@code VerifyException} with no
     * message otherwise.
     *
     * @throws VerifyException if {@code expression} is {@code false}
     */
    public static void verify(boolean expression) {
        if (!expression) {
            throw new VerifyException();
        }
    }

    /**
     * Ensures that {@code expression} is {@code true}, throwing a {@code VerifyException} with a
     * custom message otherwise.
     *
     * @param expression a boolean expression
     * @param errorMessageTemplate a template for the exception message should the check fail. The
     *        message is formed by replacing each {@code %s} placeholder in the template with an
     *        argument. These are matched by position - the first {@code %s} gets
     *        {@code errorMessageArgs[0]}, etc. Unmatched arguments will be appended to the
     *        formatted message in square braces. Unmatched placeholders will be left as-is.
     * @param errorMessageArgs the arguments to be substituted into the message template. Arguments
     *        are converted to strings using {@link String#valueOf(Object)}.
     * @throws VerifyException if {@code expression} is {@code false}
     */
    public static void verify(boolean expression, @Nullable String errorMessageTemplate,
            @Nullable Object... errorMessageArgs) {
        if (!expression) {
            throw new VerifyException(format(errorMessageTemplate, errorMessageArgs));
        }
    }

    /**
     * Ensures that {@code reference} is non-null, throwing a {@code VerifyException} with a default
     * message otherwise.
     *
     * @return {@code reference}, guaranteed to be non-null, for convenience
     * @throws VerifyException if {@code reference} is {@code null}
     */
    @CanIgnoreReturnValue
    public static <T> T verifyNotNull(@Nullable T reference) {
        return verifyNotNull(reference, "expected a non-null reference");
    }

    /**
     * Ensures that {@code reference} is non-null, throwing a {@code VerifyException} with a custom
     * message otherwise.
     *
     * @param errorMessageTemplate a template for the exception message should the check fail. The
     *        message is formed by replacing each {@code %s} placeholder in the template with an
     *        argument. These are matched by position - the first {@code %s} gets
     *        {@code errorMessageArgs[0]}, etc. Unmatched arguments will be appended to the
     *        formatted message in square braces. Unmatched placeholders will be left as-is.
     * @param errorMessageArgs the arguments to be substituted into the message template. Arguments
     *        are converted to strings using {@link String#valueOf(Object)}.
     * @return {@code reference}, guaranteed to be non-null, for convenience
     * @throws VerifyException if {@code reference} is {@code null}
     */
    @CanIgnoreReturnValue
    public static <T> T verifyNotNull(@Nullable T reference, @Nullable String errorMessageTemplate,
            @Nullable Object... errorMessageArgs) {
        verify(reference != null, errorMessageTemplate, errorMessageArgs);
        return reference;
    }

    // TODO(kevinb): consider <T> T verifySingleton(Iterable<T>) to take over for
    // Iterables.getOnlyElement()

    private Verify() {}
}
